1. Field of the Invention
The present invention is generally in the field of electrical circuits and systems. More specifically, the present invention is in the field of amplifier circuits.
2. Background Art
Switched power amplifiers such as Class-D amplifiers potentially provide efficient amplification for mobile devices. A conventional switched power amplifier, however, presents problems when used in many mobile devices like cellular telephones and mobile audio devices. Unfortunately, the driver circuit of a conventional switched power amplifier emits an electromagnetic interference (EMI) that can interfere with other mobile devices. EMI may cause a mobile device to fail Federal Communications Commission (FCC) standards, for example. Moreover, rapid output transitions may result in large transient variations as well as overshoots and undershoots in a high powered signal that drives output stage transistors in a conventional switched power amplifier. These transient variations can damage devices and severely degrade amplifier efficiency.
Neither conventional passive filters nor conventional EMI reduction circuitry has successfully addressed these shortcomings. For example, although an integrated passive filter partially limits transient variations in the high powered signal, this type of filter does not limit the EMI due to the driver circuit. Conversely, conventional EMI reduction circuitry reduces EMI but overlooks limiting the transient variations within the high powered signal. It is desirable to coherently reduce both the EMI and the large transient variations and the overshoots and undershoots of the high powered signal of a conventional switched power amplifier without significant switching losses or crowbar currents.
Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a high efficiency amplifier with reduced EMI, and which is suitable for implementation in a mobile communications device.